It adds more syntax items. It is very interesting to read 9.1 section about the compatibility with existing cookie implementation. Basically, it should overwrite an existing cookie value set by Set-Cookie if domain and path both matches.

It admits the same confusion that I had, and deprecates the Cookie2 and Set-Cookie2 headers introduced in RFC 2965!

"Prior to this document, there were at least three descriptions of

cookies: the so-called "Netscape cookie specification" [Netscape],
RFC 2109 [RFC2109], and RFC 2965 [RFC2965]. However, none of these
documents describe how the Cookie and Set-Cookie headers are actually
used on the Internet (see [Kri2001] for historical context). In
relation to previous IETF specifications of HTTP state management
mechanisms, this document requests the following actions:
1. Change the status of [RFC2109] to Historic (it has already been
obsoleted by [RFC2965]).
2. Change the status of [RFC2965] to Historic.
3. Indicate that [RFC2965] has been obsoleted by this document.
In particular, in moving RFC 2965 to Historic and obsoleting it, this
document deprecates the use of the Cookie2 and Set-Cookie2 header
fields."

The actual format is getting updated as well. Notice that now both Expires and Max-Age are supported with Max-Age overriding Expires if both exist. If not, the cookie is kept by user agent till the current session ends.